1. Field of the Invention
This invention relates to a control system for plants, which controls a plant by means of feedback control based on an adaptive control theory.
2. Prior Art
Conventionally, an air-fuel ratio control system for internal combustion engines is known, e.g. from Japanese Laid-Open Patent Publication (Kokai) No. 7-247886, in which a parameter-adjusting mechanism calculates adaptive parameters and an adaptive controller carries out feedback control of the air-fuel ratio of a mixture supplied to the engine to a desired air-fuel ratio by using the adaptive parameters. In the known air-fuel ratio control system, an air-fuel ratio sensor arranged in the exhaust system of the engine detects the air-fuel ratio of the mixture and supplies a signal indicative of the detected air-fuel ratio to the adaptive controller, which in turn determines an amount of fuel to be supplied to the engine such that the detected air-fuel ratio becomes equal to a desired air-fuel ratio. In this case, the internal combustion engine is a plant as an object to be controlled, while the amount of fuel to be supplied to the engine and the desired air-fuel ratio are a manipulated variable and a desired value to which an operation of the plant is to be controlled, respectively.
In general, the adaptive controller can exhibit a desired performance when it is designed to be adapted to an actual dead time (i.e. delay before the feedback control takes effect) which the plant and means for detecting the output from the plant possess. In the above-mentioned publication, an embodiment is described in which it is assumed that the actual dead time of the internal combustion engine and the air-fuel ratio sensor is as long as three control cycles.
If the actual dead time of the engine and the air-fuel ratio sensor increases due to a modification of the specifications of the engine and the sensor or the like, however, the orders of terms of equations applied to the adaptive controller and the adaptive parameter-adjusting mechanism are increased when the adaptive controller per se is adapted to the increased dead time, resulting in a drastic increase in the amount of arithmetic operations required to obtain the manipulated variable. This makes it difficult to carry out high-accuracy control so as to cope with an increase in the dead time unless the computer constituting the adaptive controller and the adaptive parameter-adjusting mechanism is replaced by one with higher computing performance.